Australia: The Land Where Time Began

A biography of the Australian continent 

In the Devonian Period, about 350 million years ago, a U-shaped barrier reef surrounded the Kimberleys. Its remains now form the Napier Range, known to the early white settlers as the Barrier Range. It takes the form of a U with its loop from Wyndham to Derby around the coast, and the open side 300 km inland. The reef complex, formed by the now Napier Range and the Ningbing Range near Kununurra, and the Oscar Range, skirted around the Kimberley on the northern margin of the Canning Basin.

It is regarded as one of the best-preserved reefs in the world. It was built in a shallow sea, that fringed the Kimberleys at the time, by several limestone-secreting organisms, As well as the usual corals, there were also calcareous algae and stromatoporoids, marine creatures that have since become extinct.

The Napier Range, about 100 km long, rises fortress-like about 60 m above the plain. In places they look much as they would have when they were covered by the  sea. In a section east of Yamerra Gap, the reef takes the form of dark spires 3-4 m high on a completely barren plateau, without even soil. The spires range in colour from grey to matt black, and ring when tapped with a solid object. The eeriness of this strange landscape is completed by the lack of any life apart from the small black native bees.

The Lennard River has carved a 3.5 m-long, 80 m high gorge, 100 m in places, Windjana Gorge, through the Napier Range. Large masses of conglomerate are associated with the limestone of the reef that were carried down by rivers flowing from the mountainous northern Kimberley during the Devonian.

Amphipora, (aka spagetti rock) is on the inside of the rim of the Devonian reef. On the sloping front of the reef many marine fossils have been found. Large clams, ammonoids and sponges, as well as fossils of cyanobacteria and stromatoporoids, the latter being the organism that formed the basic structure of the reef. Conodont  fossils have been utilised to divide the reef into 35 zones. After the mass extinction of the Late Devonian cyanobacteria flourished and formed large numbers of stromatolites. Regular cycles of solar radiation during the Devonian can be seen in the back-reef deposits.

Tunnel Creek, in Tunnel Creek National Park, flows through a 750m-long tunnel through the Napier Range. Geikie Gorge is also cut through the Devonian reef.

See Lloyd Hill

Sources & Further reading

1. Hellen Grasswill & Reg Morrison, Australia, a Timeless Grandeur, Lansdowne, 1981

Links

Paleomagnetism of Upper Devonian reefal limestones, Canning basin, Western Australia